Repeaters are set up in an area where signals are weakly received from a main transmitter to resolve a problem of instable signal reception and widen signal transmission coverage of the main transmitter.
FIG. 1 shows a conventional repeating system where different frequencies are used among the repeaters.
Referring to FIG. 1, a main transmitter 101 transmits a signal of a frequency A and each of repeaters 102 to 105 repeats a signal of frequencies B, C, D and E, respectively, which are different from the frequency A. The conventional repeating system uses signals of the frequencies B, C, D and E, which are different for the repeater 102 to 105 respectively. Since a plurality of frequency bands are used, many frequency resources are required and it is inefficient in the respect of using the frequency.
FIG. 2 shows another conventional repeating system where the same frequency is used among repeaters.
A main transmitter 201 transmits a signal of a frequency A and on-channel repeaters 202 to 205 repeat the signal in the same frequency A. The signals of the same frequency transmitted from the main transmitter 201 and the on-channel repeaters 202 to 205 should be individually identified for on-channel repeating.
However, when the signals of the same frequency band outputted from the main transmitter and the repeaters are different, the signals are not removed as on-channel interference signals by an equalizer or other devices in each repeater.
Also, when the signals transmitted from the main transmitter and the on-channel repeaters have a time delay longer than a predetermined level, the equalizer cannot remove the delayed signal. Therefore, the output signals of the on-channel repeater should be the same as the output signals of the main transmitter for on-channel repeating, and the time delay of two output signals should be short.
Problems of the conventional on-channel repeaters will be described with reference to FIGS. 3 to 7.
FIG. 3 is a block diagram showing a conventional RF amplification on-channel repeater.
Referring to FIG. 3, a reception antenna 301 and an RF receiver 302 receive RF signals transmitted from the main transmitter. An RF band-pass filter 303 passes only signals of a predetermined signal band in the received RF signals and a high-power amplifier 304 amplifies the band-passed RF signals. The amplified RF signal is transmitted through on-channel through a transmission antenna 305.
FIG. 4 is a block diagram showing a conventional Intermediate Frequency (IF) conversion on-channel repeater.
Referring to FIG. 4, a reception antenna 401 and an RF receiver 402 receive RF signals transmitted from the main transmitter. An IF down-converter 403 converts the received RF signals into IF signals based on a reference frequency provided by a local oscillator (LO) 408. An IF band-pass filter 404 passes the IF signals of a predetermined band. An RF up-converter 405 converts the band-passed IF signals into n RF signals based on the reference frequency provided by the local oscillator 408. A high-power amplifier 406 amplifies the RF signals and the amplified RF signals are transmitted through a transmission antenna 407.
FIG. 5 is a block diagram showing a conventional on-channel repeater employing surface acoustic wave (SAW) filter.
Referring to FIG. 5, a reception antenna 501 and an RF receiver 502 receive RF signals transmitted from the main transmitter and an IF down-converter 503 converts the received RF signals into IF signal based on a reference frequency provided by a local oscillator 508.
A SAW filter 504 passes IF signals of a predetermined band. An RF up-converter 505 converts the band-passed IF signals into RF signals based on the reference frequency provided by the local oscillator 508. A high-power amplifier 506 amplifies the RF signals and the amplified RF signals are transmitted through a transmission antenna 507.
Since the on-channel repeater of FIGS. 3 to 5 cannot remove noise and multi-path signals caused in a channel between the main transmitter and the on-channel repeater, feedback signals caused by low isolation of a transmission/reception antenna, and system noise added in an on-channel repeater system, it has a characteristic that an output signal is inferior than an input signal. Also, there is another problem in that the feedback signals generated due to the low isolation of the transmission and reception antennas restrict the transmission output power of the on-channel repeaters.
FIG. 6 is a block diagram showing a conventional on-channel repeater performing a modulating/demodulating procedure.
Referring to FIG. 6, a reception antenna 601 and an RF receiver 602 receive RF signals transmitted from the main transmitter. An IF down-converter 603 converts the received RF signals into IF signals based on a reference frequency provided by a local oscillator 611. A demodulator 604 demodulates the IF signals into baseband signals. An equalizing and forward error correction (FEC) decoding unit 605 remove noise and multi-path signals caused in a channel between the main transmitter and the on-channel repeater from the demodulated baseband signal, and feedback signals caused by low isolation of a transmission/reception antenna. A FEC decoder 606 performs coding for error correction of output signals of the equalizing and FEC decoding unit 605. A modulator 607 converts the FEC encoded signals into signals of an IF band. An RF up-converter 608 converts the IF signals into an RF signal based on a reference frequency provided by a local oscillator 611. A high-power amplifier 609 amplifies the RF signals and the amplified RF signals are transmitted through a transmission antenna 610.
Through the equalizing and FEC decoding unit, the on-channel repeater of FIG. 6 improves the multi-path and noise removing capability which is the problem of the repeater shown in FIGS. 3 to 5. However, since the on-channel repeater includes the equalizing and FEC decoding unit, it increases time delay from a microsecond unit to a millisecond unit. In addition, the transmission output power is limited when the feedback signals generated by ambiguity of a standard Trellis encoder of the FEC encoder is not removed in the repeater.
FIG. 7 is a block diagram showing a conventional on-channel repeater capable of compensating for distortion of a reception channel.
Referring to FIG. 7, an RF receiver 701 receives RF signals transmitted from the main transmitter and a down-converter 702 converts the received RF signals into signals of a desired band.
An inverse channel estimator 703 estimates an inverse of the reception channel including noise and multi-path signals caused in a channel between the main transmitter and the repeater from the converted signal, and feedback signals caused by low isolation of a transmission/reception antenna.
An adaptive filter 704 compensates for channel distortion based on inverse information of the estimated reception channel.
An up-converter 705 converts the compensated signals into RF signal and an RF transmitter 706 transmits the converted RF signals.
When the electric field strength offeedback signals (which are caused by low isolation of the transmission and reception antennas) is higher than the electric field strength of the input signal transmitted from main transmitter, the on-channel repeater of FIG. 7 does not remove distortion signals in the adaptive filter and does not estimate an inverse of the reception channel in the inverse channel estimator, thereby causing malfunction of the repeater.
Since the conventional technologies have a limitation in their removing capability of feedback signals, the conventional on-channel repeating systems have a low applicability in using a typical repeating facility and require a great deal of investment.
Therefore, it is required to develop an on-channel repeater having characteristics that the output signals of the on-channel repeater is the same as the output signals of the main transmitter, that the time delay between two output signals is small, that a characteristic of the on-channel repeater output signal becomes superior to that of the on-channel repeater input signal by removing the noise and multi-path signals caused in the channel between the main transmitter and the on-channel repeater, and that the applicability is raised and the small amount of investment is required by increasing transmission output power of the on-channel repeater by removing the feedback signals caused by the low isolation of transmission and reception antennas in the on-channel repeater.